Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 181725

Area Forecast Discussion
National Weather Service Wilmington NC
1225 PM EST Sat Nov 18 2017

A breezy warmup today will be followed by a strong cold front
late tonight and Sunday morning. The front may be accompanied
by some gusty late night showers. The front will usher in much
colder and drier air with Monday seeing the lowest daytime highs
with the possibility for a freeze or frost late Sunday night
and Monday night. Temperatures will moderate Tuesday and
Wednesday. Forecast uncertainty increases mid to late next week
as another cold front moves into the area. The weather could
turn unsettled and will hinge on the track of developing low
pressure in the Gulf of Mexico.


As of 1030 AM Saturday...Only a couple of minor tweaks to the
forecast this morning mainly with regards to the temperature
trends and cloud cover encroaching into the area from the west.
No other changes. Previous discussion follows:

Coldest temperatures of the season so far ongoing across the
area this morning. Many locations have dropped to 35 or less,
with a few, Marion, Lumberton, Kingstree, and Conway dropping to
32 or lower. The current 29 at Lumberton is the coldest
anywhere in our CWA so far this fall. These numbers are all well
below any available guidance, and while the LAV numbers and RAP
profiles have tried to "catch up," they remain too warm and
this has made the next few hours a difficult forecast. Temps are
lower than guidance likely due to cloud cover eroding faster
than forecast due to subtle ridging and downward motion aloft,
combined with weak ageostrophic drainage into the area from the
surface high centered just to the north allowing the coldest/dry
air to bleed southward into the CWA. Although high level cloud
cover is encroaching from the NW, expect this will have little
impact on temps until just before daybreak when subtle rises may
occur, and have expanded the FR.Y for nearly the entire area.
It is possible some counties may reach freeze warning thresholds
before daybreak, but confidence is low since temps should
slowly start to creep upward after 5am, so have opted not to go
with FZ.W, especially since it would be such a late decision.

The aforementioned increased cloud cover is associated with a
shortwave and vort max moving through NC this morning, which is also
creating a few sprinkles/showers upstream. This feature is progged
to weaken as it lifts ENE and do not expect any precip even in the
far NW zones this morning - with moisture manifesting only as cloud
cover instead.

Despite the very cold start, temps will rebound quickly this
morning. A warm front lifting through the Southeast ahead of a
strong cold front will push the area into a brief but strong warm
sector beginning this eve. 850mb temps are forecast to climb from
around 7C this morning, to as high as 13C this evening. This will
allow temps to soar into the low 70s most places, despite increasing
clouds both at cirrus level, and some advective stratocu and diurnal
cu in the lower levels of the column. Pushing these temperatures
upward are increasingly strong SW winds typical in the warm sector,
with forecast profiles suggesting speeds reaching 30 kt at the
top of the aftn PBL. This will potentially mix down as wind
gusts to 20-30 mph this evening, and at the same time preclude
much of an inversion after sunset so winds will remain elevated
through the end of the period.

These strong winds will become even more important tonight as a
strong cold front driven by a potent shortwave digs across the
Carolinas, moving offshore Sunday morning. Along this front an area
of convection is likely, and the question remains as to how strong
this may be, and how much QPF will occur across the area. Guidance
has slowed the FROPA, so any showers are not expected until well
after midnight, and likely won`t exit the coast until Sunday
morning. This front will encroach upon a region characterized by
very little CAPE (the most I could find was just 60 J/kg of MUCape),
but low-level winds of 50-60 kt at 2-3 kft, very strong 0-6km
shear near 60 kt, and strong but weakening 925-700mb fgen.
Parcels lifting along the front will be capped above 700mb, at
which temperatures are too warm for charge separation, and the
weak inversion above this layer will not be overcome due to the
weak instability. For this reason have left out any mention of
thunder for tonight. However, showers occurring within these
very strong low-level wind fields will support strong wind
gusts, possibly reaching 40-50 mph in any stronger echoes as
the line shifts east towards daybreak Sunday. Rainfall will be
limited across the area as showers likely weaken as they
progress quickly to the east.

With the slower FROPA, WAA will be ongoing much of tonight, so mins
will be uncharacteristically warm, dropping only to around 60 at the
coast, mid 50s inland, quite a change from the temperatures being
experienced at present!


As of 300 AM Saturday...Models have slowed and the passage of
a strong cold front should take place a few hours later than
previously thought. Current thinking is that the front will be
moving through the inland zones near daybreak Sun and then
offshore mid and late morning. The slower timing requires that
we keep a low risk for showers in the forecast a little longer,
but maintaining the theme of the last several cycles, QPF will
be light and generally a tenth of an inch or less in any one
location. Will end the risk for showers from W to E Sun morning
as quick clearing surges across the eastern Carolinas. The
coldest air will move across the area later in the afternoon and
Sun eve with 850 mb temps dropping to near zero. The 50 kt low-
level jet (2-3 kft off the surface) will be lifting out
by/during Sun morning, but cold and dry air advection will allow
for a well mixed environment and so it should be brisk on Sun
with sustained winds in the 15 to 30 mph range. A slower timing,
especially given the quick return to sunshine, should allow
temps to reach within a few degrees of normal given lows Sat
night will be well above normal.

Canadian high pressure will build to the coast Sun night and then
overhead Mon and Mon night. The strongest cold air advection
will wane overnight Sun, however, radiational cooling will
maximize as winds go light to calm. Precipitable water values
drop to just a tenth of an inch and it does not get drier than
that. Dewpoints will fall into the 20s overnight Sun and so
there will be plenty of room for temps to drop. Much of the
guidance has come in a little warmer for Sun night, but I think
they will trend lower in later runs. Frost seems like a low
risk given the magnitude of dry air, but a freeze is very much
in play for much of our inland communities, with a much lower
risk for freezing temps near the more immediate coast. Later
packages will likely be raising Freeze Watches and Freeze
Warnings for portions of the Forecast Area.

Monday will be the coolest day of the week and after a chilly
start, highs are only expected to recover into the mid to
perhaps upper 50s, even with bright sunshine with afternoon
relative humidity dropping through the 30s. Light winds on Mon
will not add a significant sting to the chill. The center of the
high will move offshore later Mon night, but will continue to
ridge back across the eastern Carolinas, keeping the column
exceedingly dry. Typically, models will show temps moderating
too quickly in these situations. Thus, the low temp forecast
undercuts model consensus in most locations. We are forecasting
lower to mid 30s with upper 30s to 40 degrees at the immediate
coast. Where the growing season is still ongoing, a widespread
frost may occur with some inland locations dipping to freezing.
The only limitation to frost development would be if dewpoints
can not recover sufficiently to bring dewpoint depressions to
within a degree or two of each other overnight. The risk for
frost will exist as you near the coast as well, but beach
communities should be spared except on an isolated basis.


As of 300 PM Friday...A split upper pattern will exist next week
across North America. A series of shortwaves moving across
southeastern Canada in the northern stream will have relatively
little impact on the Carolinas, however a pair of shortwaves in the
southern stream will have impacts on our sensible weather Tuesday
and Thursday Night into Friday.

Veering low-level winds and a warm advection pattern developing
Tuesday and Tuesday night should support the development of a
coastal trough as a weak shortwave approaches from the
southwest. There are significant model differences in the track
the shortwave takes (ECMWF from south GA to Cape Fear; GFS from
central GA to near Norfolk) which then determine how well-
developed the surface system can become. A compromise solution
should advect enough shallow Atlantic moisture back onshore for
a 20-30 percent chance of showers Tuesday night, clearing out by
Wednesday morning as the shortwave moves offshore. Cold air
will sneak south as another Canadian high moves across the Ohio
Valley. This should keep highs on Thanksgiving Day only in the
upper 50s to around 60, a good 6-8 degrees below normal.

Attention then turns to a much stronger shortwave diving into the
western and central Gulf of Mexico Wednesday night and Thursday.
Just like yesterday there are still model differences here, with the
ECMWF slower than the GFS with the eastward movement of the trough
across the Gulf of Mexico Thursday and Friday. A compromise solution
of the operational GFS, ECMWF, and their ensembles brings rain
chances into the area Thursday night through Friday Night, but given
all the uncertainty (and to better blend with surrounding NWS
offices) I`m capping PoPs no higher than 30 percent for now.


As of 18Z...High pressure is retreating farther offshore and a
belt of stronger southerly winds is beginning to move into the
central and eastern Carolinas ahead of a cold front in the lower
Mississippi Valley. This front will sweep eastward and through
the area between 08-12Z Sunday morning, accompanied by mainly
VFR cloud ceilings and scattered light showers. Increasing winds
will be the largest aviation concern.

The worst of the wind will remain just above the surface
tonight due to poor vertical mixing. Winds 500-2000 feet AGL
should increase to 35-40 knots from the south between 03-05Z
Sunday, but surface winds should only increase to 10-15 kt with
gusts to 25 kt. With the front expected to be offshore by
sunrise Sunday, post-frontal winds should veer northwesterly
with speeds gradually diminishing throughout the day.

Extended Outlook...VFR conditions are expected Sunday through
Tuesday morning. There is potential for MVFR ceilings Tuesday
night through Wednesday.


As of 300 AM Saturday...Placid marine conditions early this morning
will begin to deteriorate quickly this aftn. A warm front will lift
north through the Southeast today, ahead of a cold front which will
approach the waters late tonight. This leaves increasing SW winds
across the area, with speeds rising from 10-15 kt late this
morning, to 20-30 kt tonight. Gusts to gale force are also
likely, and may approach 40 kts at times. These strong winds
have necessitated the issuance of an SCA, which goes into effect
at 7pm this evening for all waters. After a period of low-
amplitude seas through this morning, wave heights will grow
steadily, finally peaking at 5-9 ft tonight during the period of
strongest winds. The cold front will likely be positioned just
west of the waters at the end of the period, but scattered
showers with locally higher and erratic winds are possible
tonight anywhere in the local waters.

As of 300 AM Saturday...Strong Small Craft Advisory conditions
will slowly wane through the day Sun. A strong cold front will
be moving offshore Sun morning, sweeping in a chilly and very
dry airmass. Canadian high pressure will slowly build from the
west Sun afternoon and night and then overhead Mon. The center
of the high will move offshore late Mon night.

Sustained wind speeds will be up to 25 to 30 kt Sun morning with
a few lingering gusts to gale force early, decreasing to about
20 kt by late Sun afternoon and 15 to 20 kt overnight Sun. A
further reduction in wind speeds is expected Mon, from 10 to 15
kt to 10 kt or less and these light winds will hold Mon night.
Seas will be highest Sun morning, up to 6 to 9 ft. The developing
strong offshore flow Sun should knock seas below 6 ft throughout
late in the day. Seas will subside to 3 to 4 ft Sun night and
to 2 ft or less Mon afternoon and night.

The wind direction will be shifting from SW and WSW to NW Sun
morning. NW winds will veer to N Sun night. N winds Mon will
veer to NE with easterly winds developing late Mon night.

As of 300 PM Friday...Return flow behind the departing high
Tuesday will buckle as a coastal trough develops along the
Southeast coast. Winds should turn easterly on Tuesday, then
perhaps southeasterly for a time Tuesday night as the trough
makes its closest approach to shore, but by Wednesday morning
the feature should retreat back offshore as high pressure
advances across the Ohio Valley and shifts our winds to a more
northerly direction.


MARINE...Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday
     for AMZ250-252-254-256.



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